Estimating influenza deaths in Canada, 1992-2009.

BACKGROUND: Poisson regression modelling has been widely used to estimate the disease burden attributable to influenza, though not without concerns that some of the excess burden could be due to other causes. This study aims to provide annual estimates of the mortality and hospitalization burden attributable to both seasonal influenza and the 2009 A/H1N1 pandemic influenza for Canada, and to discuss issues related to the reliability of these estimates. METHODS: Weekly time-series for all-cause mortality and regression models were used to estimate the number of deaths in Canada attributable to influenza from September 1992 to December 2009. To assess their robustness, the annual estimates derived from different parameterizations of the regression model for all-cause mortality were compared. In addition, the association between the annual estimates for mortality and hospitalization by age group, underlying cause of death or primary reason for admission and discharge status is discussed. RESULTS: The crude influenza-attributed mortality rate based on all-cause mortality and averaged over 17 influenza seasons prior to the 2009 A/H1N1 pandemic was 11.3 (95%CI, 10.5 - 12.1) deaths per 100 000 population per year, or an average of 3,500 (95%CI, 3,200 - 3,700) deaths per year attributable to seasonal influenza. The estimated annual rates ranged from undetectable at the ecological level to more than 6000 deaths per year over the three A/Sydney seasons. In comparison, we attributed an estimated 740 deaths (95%CI, 350-1500) to A(H1N1)pdm09. Annual estimates from different model parameterizations were strongly correlated, as were estimates for mortality and morbidity; the higher A(H1N1)pdm09 burden in younger age groups was the most notable exception. INTERPRETATION: With the exception of some of the Serfling models, differences in the ecological estimates of the disease burden attributable to influenza were small in comparison to the variation in disease burden from one season to another.

Transmissibility of the 1918 pandemic influenza in Montreal and Winnipeg of Canada.

BACKGROUND: The threat of 2009 pandemic influenza A (H1N1) is still causing widespread public concern. A comprehensive understanding of the epidemiology of 1918 pandemic influenza commonly referred to as the Spanish flu may be helpful in offering insight into control strategies for the new pandemic. OBJECTIVE: We explore how the preparedness for a pandemic at the community and individual level impacts the spread of the virus by comparing the transmissibility of the 1918 Spanish flu in two Canadian cities: Montreal and Winnipeg, bearing in mind that each pandemic is unique and the current one may not follow the pattern of the 1918 outbreak. METHODS: The historical epidemiological data obtained for Montreal and Winnipeg in Canada is analyzed to estimate the basic reproduction number which is the most important summary measure of transmission potential of the pandemic. RESULTS: The transmissibility of the 1918 pandemic influenza virus in Winnipeg in the fall of 1918 was found to be much lower than in Montreal based on the estimated reproduction number obtained assuming different serial intervals which are the time between onsets of symptoms in an index case and a secondary case. CONCLUSION: The early preparedness and public health control measures could suggest an explanation for the fact that the number of secondary cases generated by a primary case was significantly reduced in Winnipeg comparing to it in Montreal.